Alert “File system on control domain full”.
Solved it. This is a nice article for solving free space problems on XenServer: root disk maintenance.
I used the part for cleaning up the /var/patch directory.
xe patch-destroy uuid=<FILENAME, SUCH AS 4d2caa35-4771-ea0e-0876-080772a3c4a7>
(repeat “xe patch-destroy uuid=” command for each file with the UUID convention)